Ventilation Perfusion Relationships Flashcards
What is the alveolar gas equation?
PAO2 = PIO2 - (PaCO2/R)
What is the alveolar gas equitation used to calculate?
What inspired O2 needs to be to produce a desired alveolar and therefore arterial O2 level
(Oxygen concentration in the alveoli)
How is PIO2 calculated?
(Barometric pressure - water vapor pressure) xFiO2
What is barometric (atm) pressure?
760 mmHg at sea level
Decreases with increasing altitude
What is water vapor pressure?
Air is humidified as you inhale it
At body temperature (or anything close) it is 47mmHg
What is the FiO2?
Fraction of inspired oxygen (percent oxygen inhaled)
21% is normal (regardless of altitude)
Changes if you supplement oxygen (ex. Pt is being given 100% oxygen)
What is the FiO2 if the pt is breathing “room air”
21%
How do you determine the health of the alveoli?
Calculate the A-a O2 gradient
How is the A-a O2 gradient calculated?
PAO2 - PaO2
What is the normal value for the A-a O2 gradient?
Anything less than 20 mmHg is considered normal
Depends on age
An increase in the A-a O2 gradient indicates what?
A diffusion impairment (something is wrong at the alveoli)
What does it mean if the A-a gradient is normal but the PaO2 and PAO2 are both low?
The problem is elsewhere - not in the alveoli
e.g. pt may be hypoventilating
The intrapleural fluid is also acted on by what?
Gravity and affects the intrapleural pressure
What is the Pip at the apex of the lung?
There is less intrapleural fluid so the Pip is more negative
Alveoli are large at rest
Alveoli can only get a little larger with inspiration
What is the Pip at the base of the lung?
There is more intrapleural fluid so the pressure is less negative
Alveoli are small at rest (not as expanded by the pressure)
Alveoli can get much larger with inspiration
What is the ventilation/perfusion (V/Q) ratio?
Mathematical ratio between ventilation and perfusion over the entire lung
What is the average V/Q ratio?
0.8
4L/min / 5L/min
What does a high V/Q ratio mean?
Ventilation is high relative to perfusion (low)
What does a low V/Q ratio mean?
Ventilation is low with a high perfusion
A low V/Q ratio represents what?
A shunt - based on O2 and CO2 the blood gained nothing by the lungs as if the blood never went to the lungs in the first place
Describe a low V/Q region
Low PaO2
High PaCO2
Low pHa
High volume of blood relative to V
Describe a high V/Q region
High PaO2
Low PaCO2
High PHA
Low volume of blood
What is the V/Q ratio at the apex of the lungs (zone 1)?
High V/Q ratio leading to a higher PaO2 and a lower PaCO2
High pHa
E.g. PaO2 = 130 and PaCO2 = 28
What is the V/Q ratio in the middle of the lungs (zone 2)?
About 0.8 so “normal” and our blood gases are at expected values
What is the V/Q ratio at the base of the lung (zone 3)?
Low V/Q ratio
Low PaO2 and high PaCO2 with a low pHa
E.g. PaO2 = 89 and PaCO2 = 42
What can be done to minimize the differences in the V/Q ratios in the different zones of the lungs?
Hypoxic vasoconstriction (constriction of pulmonary vessels to redirect blood away from hypoxic regions)
